Wollastonite Mineral Overview

• Mineral name: Wollastonite
• Mineral class: Silicate (inosilicate)
• Chemical formula: CaSiO₃
• Crystal system: Triclinic
• Mohs hardness: 4.5–5
• Color range: White, gray, cream, pale brown
• Luster: Vitreous to pearly
• Streak: White
• Cleavage: Perfect in one direction, good in another
• Fracture: Splintery to uneven
• Specific gravity: Approximately 2.8–3.1
• Transparency: Translucent to opaque
• Common locations: United States, Canada, China, India, Mexico, Finland
• Uses: Ceramics, plastics, paints, metallurgy, construction materials
• Similar minerals: Xonotlite, tremolite, diopside

Physical and Optical Characteristics

Wollastonite typically occurs as bladed, fibrous, or acicular (needle-like) crystals, often forming radiating or columnar aggregates. In many deposits, it appears as massive or compact material rather than distinct crystals.

The mineral is usually white or light-colored, though gray, cream, or pale brown shades may occur due to impurities such as iron. Its vitreous to slightly pearly luster is most noticeable on fresh cleavage surfaces.

Wollastonite has a Mohs hardness of 4.5 to 5, making it relatively soft compared to many silicate minerals. It is brittle, and fracture surfaces are commonly splintery due to its fibrous structure.

Cleavage is well developed, typically showing one perfect direction and another good direction, which can produce elongated fragments when broken.

Chemical Composition and Structure

Wollastonite is composed of calcium silicate (CaSiO₃) and belongs to the inosilicate group, characterized by chains of silicate tetrahedra.

Its structure consists of single chains of SiO₄ tetrahedra linked together, with calcium ions occupying spaces between the chains. This arrangement contributes to:

• Its fibrous or bladed crystal habit
• Moderate hardness
• Cleavage characteristics

Minor substitutions, such as iron or manganese, may occur and can slightly influence color and physical properties.

Formation and Geological Occurrence

Wollastonite forms primarily in contact metamorphic environments, especially where limestone or other calcium-rich rocks are altered by heat and silica-rich fluids.

Typical formation conditions include:

• Contact metamorphism near igneous intrusions
• Interaction between calcium carbonate (limestone) and silica
• Elevated temperatures with relatively low pressure

A simplified formation reaction is:

• Calcite (CaCO₃) + Silica (SiO₂) → Wollastonite (CaSiO₃) + CO₂

Wollastonite is commonly associated with:

• Garnet
• Diopside
• Vesuvianite
• Calcite
• Quartz

These minerals are characteristic of skarn and contact metamorphic deposits.

Common Locations

Wollastonite is found in several major deposits worldwide:

• United States: New York (notably one of the largest producers)
• Canada: Ontario and Quebec
• China: Major modern producer
• India: Significant industrial deposits
• Mexico and Finland: Additional important sources

These deposits are often associated with metamorphosed carbonate rocks.

Uses and Practical Applications

Wollastonite has significant industrial importance due to its physical and chemical properties.

Major uses include:

• Ceramics: Improves strength, reduces shrinkage, and enhances thermal stability
• Plastics and polymers: Used as a reinforcing filler
• Paints and coatings: Provides durability and improves performance
• Metallurgy: Used as a flux in steelmaking
• Construction materials: Incorporated into cement and other building products

Its acicular (needle-like) structure makes it particularly valuable as a reinforcing material.

Similar and Related Minerals

Wollastonite can resemble other fibrous or light-colored silicate minerals:

• Xonotlite: Similar fibrous habit but forms under different conditions and is typically softer
• Tremolite: Amphibole mineral with fibrous crystals, but different composition and structure
• Diopside: Often associated in the same environments but typically forms more prismatic crystals
• Calcite: Softer and reacts with acid

Distinguishing wollastonite often involves:

• Observing crystal habit and cleavage
• Checking hardness
• Considering geological setting

Identification Notes for Collectors

• Look for white, fibrous or bladed aggregates in metamorphosed limestone
• Note splintery fracture and elongated crystal habit
• Check hardness (4.5–5) and cleavage directions
• Observe association with skarn minerals such as garnet and diopside
• Confirm with laboratory methods if needed in complex assemblages

Wollastonite is both an important industrial mineral and a characteristic component of contact metamorphic environments, recognized by its fibrous habit and calcium silicate composition.